I don't think you should think of it as a set of topics. Here's the thing: if you want to be able to build a backend, the only way to learn how to do it is by building a backend. So go ahead and take up a small project and learn everything along the way while you do it. Another thing, I don't know what your experience in software development is, but it doesn't make sense to know just one of backend or frontend. You could specialise in expertise once you have an idea of both, but one doesn't make sense without at least a basic understanding of the other. For example, understanding HTTP and the way the browser requests and renders a page and makes AJAX calls is vital to developing both frontend and backend effectively. Some ideas for projects:
Hope that helped, cheers :)